SILURIC FAUNA FROM SOUTHERN MICHIGAN 543 



bottom of the eastern channel of the Detroit river opposite Aniherstburg, On- 

 tario. Recent di-edgings have brought up the rock, and from it an extensive 

 fauna has been collected by the Reverend Doctor Nattress, of Aniherstburg. 

 The greatest thickness of the Aniherstburg dolomite is probably not over 20 

 or 30 feet, and a part of it is seen in the lower part of the Patrick, Gibraltar, 

 and Woolmith quarries in Michigan and the Silica and Webster quarries in the 

 Sylvania district of Ohio. 



CORRELATION OF THE BEDS 



The correlation of these beds with the beds exposed in the salt shaft at De- 

 troit is not as simple as it would at first appear. From the fact that in the quar- 

 ries cited the dolomites rest directly on the Sylvania sandstone, one would be led 

 to regard them as the equivalent of the lower or Flat Rock dolomite bed of the 

 salt shaft, which lies between the Sylvania and the Anderdon. The fossils, how- 

 ever, tell a different story. There is not a single species in common l)etween 

 the Flat Rock dolomite of the salt shaft and the Aniherstburg or Lucas beds. 

 The Lucas, however, in all its outcrops carries a fairly abundant fauna mainly 

 of gastropods, which is identical with that of the upper dolomite — 189 feet 

 thick — of the salt shaft. Moreover, the Aniherstburg dolomite, which lies 

 below the Lucas in all the quarries, contains an equal mixture of Anderdon and 

 Lucas fossils, the former often predominating. The only possible interpreta- 

 tion of this seems to be that the Lucas dolomite of the quarries represents the 

 lower part of the upper dolomite bed of the salt shaft, which therefore should 

 be referred to as the Lucas, and tliat the Aniherstburg dolomite lies between 

 the Lucas and the Anderdon, forming a transitional bed. The Aniherstburg 

 bed has not been distinguished in the salt shaft, which is not surprising when 

 the small size of the shaft is considered. 



If the above interpretation is correct, some surprising facts with reference 

 to the structural geology of southern Michigan and the adjoining areas are 

 revealed. It is first of all shown that the I^pper Moni-oe beds overlap south- 

 ward against the Sylvania sandstone as against a basal bed. This would indi- 

 cate that before the formation of the Upper ^loiiroe beds the area involved was 

 dry land. Studies of the authors on the Sylvania sandstone now in progress 

 reveal facts which point to reolian or anemoclastic origin of this sandstone. 

 Another harmonious fact is found in the thickening of the Flat Rock dolomite 

 of the salt shaft as we go northward, as shown by well records (see diagram 

 figure 1 ) . 



Another surprising fact is that in pre-Dundee time a number of \o\\ folds 

 extended across southeastern Michigan. Canada, and Ohio in a direction east- 

 northeast. A synclinal trough extended through the center of Grosse isle, past 

 the Gibraltar, Flat Rock and Woolmith quarries, probably a little to the north 

 of all of these. An anticlinal axis extended parallel to this through the Ander- 

 don quarry and through the region where now are situated the Sylvania sand- 

 pits south of Scofield. A second anticlinal axis, also parallel or approximately 

 so to the others, extended through the region of the present Sibley quarries, or 

 somewhat to the north. Peneplanation during Lower Devonic time truncated 

 these low folds, so that on the deposition of the mid-Devonic Dundee the base 

 of this formation came to rest on different members of the Upper Siluric. 

 Thus at the Anderdon quarry the bed exposed by the truncation of the anti- 



